The invention refers to an apparatus for filtering thermoplastic synthetic plastic material, having a screw bearingly supported within a housing for rotation around its longitudinal axis for the transport and preferably also plasticizing of the synthetic plastic material that is supplied to the screw into the housing through an intake opening and leaves the housing through an exit opening, for example a nozzle, wherein the screw is subdivided into two sections disposed one behind the other in axial direction and wherein a filter provided with penetration openings for the synthetic plastic material is disposed in that flow of the synthetic plastic material that is disposed between the two sections.
An apparatus of this kind has become known (EP 600,924 B). Within this known construction the two screw sections are separated from each other by a sealing section that is by-passed by the processed synthetic plastic material via a by-pass leading out of the housing. A filter is arranged in this by-pass by which impurities contained in the thermoplastic synthetic plastic material to be processed are retained. This known apparatus is well suited for fine-grained impurities if they do not constitute too high an impurity portion within the synthetic plastic material. If, however, the synthetic plastic material to be processed contains coarser impurity fractions, for example small stones or metal parts, such as bottle caps, paperclips, wire stitches or the like, the filter and the filter channels of the known apparatus become quickly blocked, so that the filter cannot accomplish its function.
The invention has as one of its objects to improve a construction of the initially described kind so that filtering of coarse impurities can be done reliably and at a low price without the filter being frequently changed. The invention solves this task by disposing the filter within the housing between the two screw sections so that it subdivides the interior of the housing and is connected for common rotation with at least one of the screw sections, and with a scraper carried by the housing engaging the upstream side of this filter, which scraper scrapes off coarse impurities from the filter when it is rotated and conveys these impurities into a collecting space in connection with the interior of the housing. This filter retains the coarse impurities so that the filter is passed only by the synthetic plastic material and fine impurities contained therein, if any. If the latter impurities are undesirable, a fine filter, for example of the initially described construction or disposed at the end of the screw, can follow.
By continuously scraping off the coarse impurities with the scraper, the upstream side of the inventive coarse filter is kept free from adsorption and deposits so that coarse impurities also do not block the penetration openings of the filter, but are compulsorily continuously conveyed off into the collecting space. When the volume of this collecting space is sufficiently dimensioned, the apparatus can operate for a long time without there being a danger of a breakdown because of blocking of the filter.
It is known within an extruder for synthetic plastic material (EP 689,918 A) to dispose a stationary fine filter within the housing between a supply screw and the extruder nozzle for the separation of impurities contained in synthetic plastic material, and to scrape off the impurities collecting on the upstream side of the fine filter by means of a scraper rotating together with the screw. For the filtering of coarse impurities, this apparatus is not useful.
According to a further embodiment of the invention, the filter is disposed at a member separate from the screw sections or constitutes such a member. This is advantageous when manufacturing and, if desired, enables one to change over already existing plants in the sense of the invention.
A particularly favorable construction consists within the spirit of the invention in that the filter is provided with a shaft section constituting the separate member, being disposed between the two screw sections and being provided with an annular flange having penetration openings, this shaft section being connected for common rotation on at least one of its front ends with the neighboring screw section. This results in a simple, easily exchangeable and reliable construction. Connection for common rotation can be made by a screw section, which has the advantage that this connection, if desired, can easily be detached at any time, but if the direction of rotation of the screw sections and of the thread direction of this screw connection are efficiently chosen, in operation the screw connection is automatically tightened so that an undesired detachment is not possible.
A particularly favorable construction consists within the spirit of the invention in that the shaft section at the upstream side of the filter has a lesser diameter than at the downstream side. Thereby, at the upstream side of the filter there is a space for the impurities scraped off by the scraper.
Further, advantages are obtained if the penetration openings of the filter have a cross-section that increases in the flow direction of the synthetic plastic material. This avoids solid bodies entering into a penetration opening can be crammed in the penetration opening, thereby blocking the filter.
Another suitable construction consists within the spirit of the invention in that the scraper is provided with a scraping edge engaging a cylindrical section of the filter, particularly a cylindrical section of the shaft section, which cylindrical section immediately neighbors the upstream side ends of the penetration openings. Thereby, the coarse impurities are compulsorily conveyed off in a continuous and reliable manner.
A preferred embodiment of the invention consists in that the housing has a central section surrounding the separate member and being detachably connected to its neighboring housing sections so that after dismounting this central housing section, the separate member carrying or constituting the filter can be put off the housing. Therefore, the filter or the member constituting or carrying it, respectively, constitutes an easily exchangeable member, which results in advantages when replacing or cleaning.
Within the invention, the scraper can be disposed within a tube connected laterally to the housing, in particular to its detachable central section, which tube with its interior constitutes at least a portion of the collecting space. This results in a low-cost construction because the members necessary for mounting the scraper at the same time serve for confining the collecting space. Within this, it is suitable to mount the scraper adjustably within the tube in a direction perpendicular to the axis of the screw, in order to enable one to adapt the position of the scraper to manufacturing tolerances and to wear occurring during operation. In order to enable one to easily exchange the scraper, when necessary, within the spirit of the invention the tube is detachably connected to the housing, in particular to its detachable central section. Further, it is suitable if the tube has a detachable closure cap, in particular a screw cap, at its end opposite the housing, because after detaching this closure cap the collecting space confined by the tube is accessible for emptying or cleaning. The collecting space can be provided with a closure valve so that a preferably automatic emptying of the collecting space is possible during operation.